Process control systems require the accurate measurement of process variables. Typically, a sensor in the form of a primary element senses the value of a process variable and a transmitter develops an output having a value that varies as a function of the process variable. For example, a level transmitter includes a primary element for sensing level and a circuit for developing an electrical signal representing sensed level.
Knowledge of level in industrial process tanks or vessels has long been required for safe and cost-effective operation of plants. Many technologies exist for making level measurements. These include buoyancy, capacitance, ultrasonic and microwave radar, to name a few.
In one form, a through air measurement, such as ultrasonic or microwave radar, comprises an antenna, horn or the like which extends into the process vessel. The horn is hollow and exposed to the materials in the vessel. A seal is generally provided between the horn and the electric circuits.
In another form of level measurement, a guided wave radar transmitter uses a coaxial probe which functions as an electronic transmission line into the process vessel. The probe includes a hollow sleeve that extends into the process vessel. Similarly, a capacitance probe includes a hollow sleeve.
With these and other measurement instruments, the primary element may be adversely effected by process build-up. To clean the primary element, it may be necessary to remove the primary element from the process vessel. However, this may be undesirable if the vessel is closed and the primary element provides a process seal.
Thus, it is advantageous to provide a means for cleaning the primary element without removing the primary element from the process vessel. One known technique uses a port opening directly into the hollow interior of the primary element. However, this may not always be effective in removing contaminants.
The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.